Implements REQ-UI-ITEM-ICON and REQ-UI-RECIPE-ICON. Adds an optional
recipes.toml `icon` field (an item id; defaults to the recipe's first
output). New ItemIconCache loads per-item SVGs from icons/items and
caches pixmaps per target pixel size, shared by:
- the Miner/Assembler recipe-selection dialog, which now shows icon-only
option buttons (name-caption fallback when no icon file exists), and
- GameWorldView belt/port item rendering via a shared drawWorldItem
helper (colored-square fallback preserved).
Shipyard schematic dialog is unchanged. No icon art is shipped; every
path falls back cleanly when an item has no SVG.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc
